In primary schools in England, programmes of Sex and Relationships Education (SRE) are rare. Provision has been judged as requiring improvement in over one-third of these schools at a time when statutory provision has been mandated by the government. The aim of this study was to examine the early implementation of Spring Fever, a programme of primary school SRE, in terms of reach, fidelity, dose, recruitment and context. Data were collected through a teachers' feedback form (n = 10), teachers' focus group (n = 9); a parent diary (n = 7), parents' interviews (n = 5), parent feedback (n = 41) and pupil feedback (n = 24). Reach was high with few pupils withdrawn. Fidelity to the lesson plans and achievement of learning objectives was high for most school years. Pupils enjoyed the programme and responded well. It was clear that learning had occurred but that pupils had been uncomfortable with some topics. Teachers were largely positive about the programme. Parents felt that it was age-appropriate, informative and handled sensitively. This is the first process evaluation of primary school SRE. Detailed descriptions of process evaluation are uncommon in the literature but this is vital for identifying issues pertinent to the future roll-out and evaluation of programmes.